Most Melbourne homes benefit from roof ventilation, and new homes now need it. Since 1 May 2024, Victoria has applied the National Construction Code’s condensation rules, which require new homes in climate zones 6, 7 and 8, Melbourne included, to have a ventilated roof space. Ventilation moves out hot air in summer and moist air in winter, which protects the roof timbers, insulation and ceilings from condensation and mould.
The short answer
- Roof ventilation removes heat in summer and moisture in winter
- New homes in climate zones 6, 7 and 8 must have a ventilated roof space under the NCC
- Victoria adopted the condensation rules on 1 May 2024
- Older tiled roofs without sarking breathe through the gaps between tiles
- Signs you need it: damp patches without rain, mould, musty smells, a very hot roof space
Why roof ventilation matters
A roof space works hard in both seasons. In summer, CSR Bradford notes an unventilated roof space can reach around 70°C, and that heat soaks through the ceiling and makes air conditioners work harder. In winter, warm moist air from showers, cooking and breathing rises into the roof space and condenses on the cold underside of the roof, wetting timbers and insulation and feeding mould.
Modern homes are better sealed and better insulated than older ones, which saves energy but traps more moisture. That is why the building code now deals with condensation directly.
Sources: CSR Bradford: The ultimate guide to roof ventilation.
What the building code requires
| Roof pitch | Low-level (eave) openings | High-level openings |
|---|---|---|
| Under 10° | 25,000 mm²/m at each of two opposing ends | Not separately specified |
| 10° to under 15° | 25,000 mm²/m | 5,000 mm²/m |
| 15° to under 75° | 7,000 mm²/m, plus 18,000 mm²/m for cathedral ceilings | 5,000 mm²/m |
Part 10.8 of the NCC 2022 Housing Provisions applies in climate zones 6, 7 and 8; Melbourne is in zone 6 on the ABCB climate zone map. Openings are measured per metre of the roof’s longest horizontal dimension, and high-level openings must be at the ridge or within 900 mm of it. The roof space must be at least 20 mm high. Concrete roofs, roofs made of structural insulated panels and roofs built to bushfire attack level FZ are exempt. A roof space directly under the tiles of an unsarked tiled roof is an accepted alternative to vents.
The Victorian Building and Plumbing Commission confirms the provisions were adopted in Victoria on 1 May 2024, for new houses and similar buildings. They do not require existing homes to be upgraded, but they are a good benchmark if you are re-roofing or have condensation problems.
Sources: NCC 2022 Housing Provisions Part 10.8: Condensation management; BPC: Condensation management guidance (GE-15).
Tiled roofs and ventilation
Many older Melbourne tiled roofs were laid without sarking, the reflective membrane under the tiles. Air moves through the small gaps between the tiles, so the roof space breathes naturally, which is why the code accepts an unsarked tiled roof space as ventilated. Newer and re-roofed homes usually have sarking, which keeps out dust and wind-driven rain but also closes those gaps, so they rely on vents instead.
If you are planning a roof replacement with new sarking, plan the ventilation at the same time. It is far easier to install vents while the roof is open.
Damp ceiling but no obvious leak?
We will check the roof and roof space and tell you whether it is a leak, condensation, or both.
Types of roof ventilation
Wind-driven ventilators
The spinning turbines often called whirlybirds. They use no power and still draw some air in calm conditions as hot air rises.
Powered ventilators
Mains or solar-powered fans. CSR Bradford says one powered vent can do the work of several wind-driven ones.
Static vents
Fixed vents with no moving parts, often used over wet areas such as bathrooms and laundries.
Eave vents
Low-level openings that let fresh air in so roof vents have something to draw through. Bradford suggests about four eave vents per roof vent.
As a rule of thumb, CSR Bradford suggests one natural roof vent for every 90 square metres of roof space, with eave vents to supply the air. Your installer will size it to your roof and the code.
Sources: CSR Bradford: The ultimate guide to roof ventilation.
Signs your roof space needs ventilating
- Damp patches or drips on the ceiling in cold weather with no rain
- Black mould spots on ceilings, especially in bathrooms and bedrooms
- A musty smell in upstairs rooms or wardrobes
- Water droplets or dark staining on the underside of the sarking or roof timbers
- Wet or compressed insulation
- A roof space that is unbearably hot in summer
Condensation is often mistaken for a roof leak. If you have damp patches but we cannot find a fault in the roof, ventilation is usually the next thing to check. Our guide to what to do when your roof leaks covers the other causes.
Installing roof vents on a tiled roof
On a tiled roof, a vent is fitted by removing tiles, cutting and flashing the opening, and refitting tiles around a base or flashing designed for the tile profile. Done badly, a new vent becomes a new leak. It needs to be sealed to the tiles, with any sarking cut and dressed so water runs over it, not into the roof space.
WorkSafe Victoria’s compliance code for housing construction puts edge protection such as guard railing ahead of harnesses and ladders when controlling the risk of falls from a roof. Our crews set up our own temporary safety rail, ladder guards and, where there are old skylights, safety nets before anyone works on the roof. Every crew member holds a working at heights qualification.
Other ways to reduce condensation
- Use exhaust fans in bathrooms and kitchens, and make sure they vent outside rather than into the roof space
- Run exhaust fans during and after showers and cooking
- Keep insulation dry and in place, without gaps over the ceiling
- Fix leaks quickly so the roof space is not damp to begin with
Common questions
Does my home need roof ventilation?
Most homes benefit from it. New homes in climate zones 6, 7 and 8, including Melbourne, must have a ventilated roof space under the NCC condensation provisions adopted in Victoria on 1 May 2024.
Do existing homes have to add roof vents?
No. The provisions apply to new houses and similar buildings. They are still a good benchmark if you are re-roofing or have condensation problems.
How many roof vents do I need?
It depends on the roof. CSR Bradford suggests about one natural vent per 90 square metres of roof space, with roughly four eave vents per roof vent. The NCC sets minimum opening areas by roof pitch.
Do tiled roofs need vents?
An unsarked tiled roof breathes through the gaps between tiles and is accepted by the code as ventilated. Tiled roofs with sarking generally need vents.
Can condensation look like a roof leak?
Yes. Damp patches that appear in cold weather without rain, or mould on ceilings, often come from condensation in the roof space rather than a leak.
Are whirlybirds worth it?
Wind-driven ventilators are a low-cost way to move air without power. Powered vents move more air, and eave vents are needed so either type has fresh air to draw through.
Related
Sources: NCC 2022 Housing Provisions Part 10.8: Condensation management; BPC: Condensation management guidance (GE-15); CSR Bradford: The ultimate guide to roof ventilation.
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